The present invention relates to a rear projection screen in which the image projected upon the screen can be observed by a viewer on the side opposite to the projector of a projection type color television system, moving system or the like.
In order to observe an enlarged television or motion-picture image in a lighted room, a rear projection or translucent screen consisting of a Fresnel lens and a lenticular lens has been generally used. In order to prevent the degradation of the contrast of the projected image, that is, in order to prevent the reflection of the undesired ambient light, the non-transmittance region (to be described in detail hereinafter) of the viewed surface of the lenticular lens is covered with masking stripes. The masking stripes are disposed (a) at the flat regions; (b) at the projected or protruded regions; and (c) at the recessed regions. In the case of covering the flat non-transmittance regions, the direct or transfer printing process is used. In the case of covering the projected or protruded regions with the masking stripes, the direct or transfer printing process or coating process is used. In the case of covering the recessed regions with the masking stripes, the coating process is used. It has been reported that the process for covering the projected or protruded regions with the masking stripes is most preferable because of the easiness with which the alignment between the masking stripes and lenticular convex lens elements is carried out and because it is rather simple to form the masking stripes. As compared with the process for covering the flat non-transmittance regions with the masking stripes, the process for covering the projected or protruded regions with the masking stripes is preferable because, in the case of the former, it is difficult to align the masking stripes with the lenticular convex lens elements. Furthermore, the process for covering the recessed regions with the masking stripes is not reliable in practice.
In the case of the rear projection or translucent screen in which masking stripes are formed over the projected or protruded regions, the projected or protruded regions are almost rectangular in cross section so that their side surfaces are substantailly perpendicular to the flat bottoms of the valleys between the adjacent projected or protruded regions and the masking stripes are formed over the tops of the projected or protruded regions. As a result, a light ray incident on a lenticular convex lens element at a point in the vicinity of the boundary between the adjacent lenticular convex lens elements is refracted at an angle with respect to the optical axis of the lenticular convex lens element and is intercepted by the perpendicular side surface of one of the projected or protruded regions. If the area of each masking stripe is increased, that is, the width of each protruded or projected region is increased in order to prevent the undesired reflection of ambient light rays, the end of each masking stripe may intercept the light rays having passed through the lenticular lens. Thus, there exists a limit in the increase of the area of each masking stripe. As a result, the conventional lenticular lenses cannot satisfactorily prevent the undesired reflection of ambient light rays, so that a clear projected image cannot be observed.
In addition, in the conventional rear projection or translucent screens, the masking stripes are formed in opposed relationship with the boundaries between the lenticular convex lens elements. In the center portion of the lenticular convex lens, the light rays from the projector, in general, enter each lenticular convex lens element in parallel with its optical axis. The light rays parallel with the optical axis of each lenticular convex lens element are focussed on the axis. Accordingly, the light rays incident on the center portion of the lenticular convex lens are not intercepted by the side surfaces of the projected regions or by the masking stripes.
However, in the circumferential portions of the lenticular lens, the light rays from the projector, in general, enter each lenticular lens element obliquely with respect to its optical axis thereby to be focussed on a point deviated from the optical axis. Accordingly, the light rays having passed through the lens element are partially intercepted by the side surfaces of the projected or protruded regions. As a result, the brightness of the projected image is decreased.
In the case of the projection type color television receivers in which red, green and blue light projectors are juxtaposed in the horizontal direction, the angles of incidence of red, green and blue light rays are different from one another, so that the amount of light rays intercepted by the masking stripes is different, especially in the peripheral portions of the lens, depending on the kind of light rays. Consequently, the three primary colors are not correctly mixed. Thus the positions of the masking stripes must be determined depending upon the positions of the corresponding lenticular convex lens elements.
In the conventional rear projection screens, a masking ink adheres to only the tops of the projections and does not adhere to the side surfaces of the projected or protruded regions when the ink is applied to the projections. Therefore, the side surfaces thereof cannot prevent ambient light in the lateral direction from being reflected on the side surfaces. When a viewer watches such a screen obliquely (in the lateral direction), the contrast of image is decreased.
Furthermore, there is a problem that a lenticular lens is apt to be formed in a manner such that the center portion of the lens is thicker than its circumferential portions when it is molded. Accordingly, the radii of the lenticular convex lens elements must be varied depending on the positions of the elements in the lens sheet in order to prevent the light rays coming out of the lens elements from being intercepted by the masking stripes.